Business Segments (Details) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2022 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2022 USD ($) Segments	Jun. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments				2
Operating Revenue		$ 373.1	$ 335.6	$ 756.6	$ 674.8
Net Income Attributable to ALLETE		37.6	27.9	103.9	79.7
Assets		6,839.0		6,839.0		$ 6,435.0
New Energy
Business Segments [Line Items]
After Tax Impact for Purchase Price Accounting				4.0
Business Combination, Acquisition Related Costs				$ 3.0
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments				3
Operating Revenue		308.7	290.4	$ 637.7	583.4
Net Income Attributable to ALLETE		29.6	21.5	81.1	66.5
Assets		4,370.9		4,370.9		4,289.4
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue		40.6	35.4	96.6	81.0
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue		44.4	40.7	92.7	80.4
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue		10.0	11.5	22.1	24.1
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue		150.6	139.1	298.4	268.1
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue		36.9	37.3	77.9	75.7
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		26.2	26.4	50.0	54.1
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		21.2	21.6	49.4	42.5
Net Income Attributable to ALLETE		5.8	5.1	22.3	12.5
Assets		1,834.9		1,834.9		1,719.4
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue		18.9	18.7	44.3	36.8
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue		2.3	2.9	5.1	5.7
Corporate and Other [Member]
Business Segments [Line Items]
Operating Revenue		43.2	23.6	69.5	48.9
Net Income Attributable to ALLETE	[1]	2.2	1.3	0.5	0.7
Assets		633.2		633.2		$ 426.2
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue		21.4	21.4	44.0	43.1
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		7.3	2.2	11.0	5.8
Corporate and Other [Member] | Sale of Renewable Development Projects
Business Segments [Line Items]
Operating Revenue		$ 14.5	$ 0.0	$ 14.5	$ 0.0

[1] Net Income in 2022 includes $4.0 million after-tax of purchase price accounting adjustments and $3.0 million after-tax of transaction costs related to the acquisition of New Energy.